Phase change memories (PCMs) are a new generation of non-volatile memories in which, in order to store information, the characteristics of materials having the property of switching between phases with different electrical characteristics are exploited. These materials may switch between a disorderly/amorphous phase and an orderly crystalline or polycrystalline phase; different phases are characterized by different values of resistivity and are consequently associated to different values of a data item stored. For instance, it is possible to use elements of Group VI of the periodic table, such as tellurium (Te), selenium (Se), or antimony (Sb), referred to as “chalcogenides” or “chalcogenic materials”, to form phase-change memory elements. In particular, an alloy made up of germanium (Ge), antimony (Sb), and tellurium (Te), known as GST (having chemical composition Ge2Sb2Te5) currently finds wide use in such memory cells.
Phase change in a memory element may be obtained by locally increasing the temperature of the cell of chalcogenic material, through resistive electrodes (generally known as “heaters”) arranged in contact with the regions of chalcogenic material.
Access (or selection) devices (for example, bipolar or MOS transistors) are connected to the heaters so as to enable selective passage of the programming electric current (also known as writing electric current) through them. This electric current, by the Joule effect, generates the temperatures required for phase change.
In particular, when the chalcogenic material is in the amorphous state, and thus has a high resistivity (the so-called RESET state), it is necessary to apply a current/voltage pulse (or a suitable number of current/voltage pulses) of a duration and amplitude such as to enable the chalcogenic material to cool slowly. Subjected to this treatment, the chalcogenic material changes state and switches from the high-resistivity state to a low-resistivity state (the so-called SET state). Conversely, when the chalcogenic material is in the SET state, it is necessary to apply a current/voltage pulse having an appropriate duration and a large amplitude so as to cause the chalcogenic material to return into the high-resistivity amorphous RESET state.
During reading, the state of the chalcogenic material is detected by applying a voltage sufficiently low as not to cause a sensible heating thereof, and then reading the value of the current flowing in the memory cell through a sense amplifier. Given that the current is proportional to the conductivity of the chalcogenic material, it is possible to determine in which state the material is, and consequently determine the data item stored in the memory cell.